Why data on guns can't be trusted

The Umpqua Community College shooting has raised questions about the number of guns in the U.S. But data on this subject are unreliable.

Here's why:

U.S. law explicitly prohibits law enforcement agencies from collecting or releasing statistics that could provide an accurate picture of gun ownership. The Bureau of Alcohol, Tobacco, Firearms and Explosives is barred from releasing gun traces to the public and can't require gun dealers to turn over inventories. The FBI, which is in charge of federal background checks, is required to destroy purchaser records within 24...